The present invention relates to a material to be used in manufacturing a spent fuel storage member comprising a cask or a rack. Such a spent fuel storage member is used for accommodating and storing spent nuclear fuel assembly after burning. This invention also relates to a method of manufacturing such a material and the spent fuel storage member manufactured from such material.
The nuclear fuel assembly no longer usable after burning in the final stage of nuclear fuel cycle is called a spent nuclear fuel. This spent nuclear fuel is stored and managed at a storage facility until the time of reprocessing. For example, in the stage method using fuel pool, SUS racks bundling square pipes are submerged in the pool, and spent fuel assemblies are put in the square pipes, so as to satisfy the requirements of cooling effect, shielding effect, and subcriticality.
Recently, boron is often added to stainless steel to manufacture the square pipes that make the racks. Conventionally, a neutron absorber is provided between the square pipes of the racks. However, such a neutron absorber is not required when square pipes made from boron-added-stainless steel are used. Accordingly, gap or distance between adjacent square pipes can be reduced and the number of square pipes that can be inserted in the pool pit can be increased. Resultantly, the storage capacity of spent fuel assemblies can be increased.
Such square pipes may be applied in various storage systems such as cask, horizontal silo, pool, and board. However, even when manufacturing the pipes, for example, for only the racks, the number of pipes to be manufactured is great. Accordingly, a technology with which a large number of square pipes can be efficiently manufactured is in demand. Further, to absorb the neutrons generated from the spent fuel assemblies securely the structure of the square pipes is required to be sturdy.
Beside the racks made from the square pipes, racks made form flat plates are also used to store the spent fuel assemblies. Accordingly, a technology with which a large number of these flat plates can be efficiently manufactured is also in demand. Further, the spent fuel assemblies obtained from the pressurized water reactor (PWR) are heavy. Therefore, the racks that accommodate and store such assemblies are required to be still stronger.
It is an object of the present invention to provide a material by the use of which stronger spent fuel storage members can be manufactured in large number.
The aluminum composite powder according to one aspect of the present invention is obtained by employing mechanical alloying, and dispersing neutron absorber, and third particle composed of oxide, nitride, carbide or boride ground by mechanical alloying in aluminum matrix.
The neutron absorber dispersed in the aluminum matrix acts to block sipping in the crystal grain boundary, and reinforce the material. The third particle composed of oxide or the like is dispersed in the aluminum matrix, and further promotes the blocking action of slipping of crystal particles, so that the material strength is further heightened. It is preferable that the content of the dispersed third particle is 0.1% by weight or more to 30% by weight or less. Further, it is preferable that the mean particle size of the third particle added in the aluminum matrix is 0.01 xcexcm or more to 10 xcexcm or less. Further, it is preferable that, in the aluminum composite powder, the content of the neutron absorber is 1% by weight or more to 20% by weight or less. Further, it is preferable that the mean particle size of the neutron absorber powder to be added is preferred to be 0.01 xcexcm or more to 100 xcexcm or less.
The manufacturing method of aluminum composite powder according to another aspect of the present invention comprises the steps of mixing aluminum powder as matrix material, neutron absorber, and third particle composed of oxide, nitride, carbide orboride, and dispersing the ground neutron absorber and third particle in the aluminum matrix by mechanical alloying of the mixed powder.
By mechanical alloying, the aluminum is crushed, plaited and flattened. Besides, the neutron absorber such as B is also finely ground by mechanical alloying, and is uniformly dispersed in the flattened aluminum matrix. Finally, these flat particles are bound to form ordinary particles. As a result, crystal slipping of aluminum is prevented and a sufficient strength is obtained, and in the invention, moreover, the third particle composed of oxide or the like is finely ground, and dispersed in the aluminum matrix. The dispersed third particle is considered to promote the blocking actin of crystal slipping. The aluminum composite powder manufactured in this method comes to have a very large strength.
Further, the steps of forming an oxide film preliminarily on the surface of the aluminum powder, and mixing the oxide film formed on the surface of the aluminum powder as the oxide into the composite powder by mechanical alloying may be added. When using the oxide as the third particle, the oxide is not added separately, but is preliminarily formed as an oxide film on the surface of the aluminum powder, and this oxide film is peeled and ground at the time of mechanical alloying, and is dispersed in the matrix as third particle. Thus, it can save the labor of manufacturing the third particle or the step of adding it, so that the aluminum composite powder can be manufactured easily.
The aluminum composite material according to still another aspect of the present invention is obtained by containing neutron absorber and third element composed of oxide, nitride, carbide or boride, in aluminum matrix. As mentioned above, by dispersing fine neutron absorber and third particle uniformly in the aluminum matrix, they are effective to block aluminum grain boundary slipping and heighten the strength of the material. By sintering such aluminum matrix powder, the neutron absorber and third element are contained in the material, so that the strength may be dramatically improved. The sintering methods include atmospheric sintering, vacuum sintering, discharge sintering, and others, and preforming may be also done before the sintering process. Such aluminum composite material may be used in the basket for containing the spent fuel assemblies. This basket is composed by setting up square pipes or, alternately combining plate members.
In the aluminum composite material, it is preferable that the content of the third particle is 0.1% by weight or more to 30% by weight or less. Further, it is preferable that the mean particle size is 0.01 xcexcm or more to 10 xcexcm or less. Further, it is preferable that the content of the neutron absorber is 1% by weight or more to 20% by weight or less. More favorable results will be obtained when the mean particle size of the neutron absorber powder to be added is 0.01 xcexcm or more to 100 xcexcm or less. The reasons of these effects are explained in the following description of the embodiments.
The manufacturing method of spent fuel storage member according to still another aspect of the present invention comprises the steps of mixing aluminum powder, neutron absorber powder, and third element composed of oxide, nitride, carbide or boride, preforming the mixed powder, canning the preformed material, and sintering the canned preformed material.
First, aluminum powder, neutron absorber, and third particle are mixed, and the mixed powder is preformed. By preforming, variance of forming density can be suppressed. Successively, the preformed material is canned, and sintered. Thus are prepared billets before forming the spent fuel storage member. To take out billets from the can, the outside or end of the can is ground. Preferably, the sintering process is done by hot pressing or hot isostatic pressing (HIP) method as a fourteenth aspect. Besides, pseudo-HIP or atmospheric sintering may be also employed. Spent fuel storage members include square pipes for composing basket or plate members for composing flat plate rack.
The manufacturing method of spent fuel storage member according to still another aspect of the present invention comprises the steps of mixing aluminum powder, neutron absorber powder, and third element composed of oxide, nitride, carbide or boride, and atmospheric-sintering or vacuum-sintering the mixed powder.
Thus, the canning step may be omitted and the atmospheric sintering or vacuum sintering may be performed. By omitting the canning step, machining such as grinding of outside is not required after atmospheric sintering or vacuum sintering. Hence, billets can be manufactured easily. Preferably, the sintering process is done by vacuum hot pressing as a sixteenth aspect. Besides, vacuum pseudo-HIP may be also possible. By executing the sintering process by vacuum hot pressing, inexpensive and high-quality spent fuel storage members may be manufactured.
The manufacturing method of spent fuel storage member according to still another aspect of the present invention comprises the steps of mixing aluminum powder and neutron absorber powder, a step of forming a preformed material by cold isostatic pressing, and discharge-sintering the preformed material.
By discharge-sintering the preformed material, sintering can be performed in a shorter time as compared with ordinary sintering. Accordingly, the spent fuel storage members can be manufactured efficiently. Besides, since the canning is omitted, outside grinding or machining is not required, and spent fuel storage members can be manufactured at lower cost. Preferably, the discharge sintering process is done by discharge plasma sintering as an eighteenth aspect. By the energy of discharge plasma sintering, the passive film of aluminum is removed, and favorable sintering is possible. Aside from discharge plasma sintering, heat plasma sintering maybe also applied.
Further, a step of forming a square pipe or a plate material for composing a square pipe by extrusion forming may be added. Also, a step of forming a bar to be inserted into a guide tube of spent fuel by extrusion forming may be added. Thus, in these methods, by extruding the manufactured billets, square pipes or bars can be manufactured easily as the spent fuel storage members as the nineteenth and twentieth aspects. Incidentally, the square pipes may be directly formed by extrusion, or formed by welding after extruding plate members.
Further, the mixing process may be performed by mechanical alloying. The mechanical alloying may be performed using the available boring mills. The particles gradually become flat when the aluminum powder is gradually plaited in the boring mill. The neutron absorber and third particle are ground by the boring mill, and become considerably smaller as compared with the initial mean particle size, and are kneaded and dispersed into the aluminum matrix. After mechanical alloying, the flat particles finally become particles containing the neutron absorber and third particle. Thus, the neutron absorber can be dispersed finely and uniformly, and the mechanical strength of spent fuel storage member can be enhanced.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.